TPVs are two-phase systems wherein cured rubber particles are finely dispersed in a thermoplastic phase. The mixing temperature must be high enough to melt the thermoplastic phase and also cure the rubber. The curing of the rubber phase occurs under conditions of dynamic vulcanization (curing the rubber during melt mixing), contrary to static curing that typically occurs in a rubber mold. Shearing must continue to be applied to prevent the agglomeration of the rubber particles since small rubber particle size is critical in obtaining a product with high performance. For TPVs to have good performance the following properties are desired: (a) the surface energies of the two phases must match, (b) the molecular weight between inter-chain entanglements in the rubber must be low, (c) thermoplastic should have crystallinity, (d) the rubber should be cured at the mixing temperature, and (e) both phases must be stable at the mixing temperature.
TPVs are processed by techniques commonly used in the plastics industry such as injection molding which makes their fabrication more efficient and cost-effective than thermosets. TPVs have non-Newtonian flow properties and their viscosity is very shear dependent. At low shear rates, their viscosity increases, flow diminishes, and they have a high retention of melt integrity and shape retention when cooled. As the shear rate increases, they become more fluid and can be more rapidly injected into a mould.
Thermoplastic elastomers find many applications, for example in coatings, adhesives and in molded and extruded parts. The latter are valued for their toughness and impact resistance, and find application in automotive parts, mechanical parts, electrical parts and other uses. Specific applications include: seals, wire covers, fuel lines and hoses, cold-air intake tubes, and CVJ boots, pedals, grips, wipers, pipe seals, electrical moldings, as well as injection molded housing and cabinetry for electronic applications.
Improvements in properties are being constantly sought, and often for this purpose polymeric materials are mixed or blended. EP-A1-0 364 859 relates to vulcanizable rubbery compositions containing a polyamide, a partially hydrogenated nitrile rubber and curatives in the nitrile rubber. The partially hydrogenated nitrile rubber, admixed with a curing agent, was gradually added to molten polyamide, with mixing. It is stated that it is preferred to use a polyamide having a low melting point, such as nylon 12. In a preferred embodiment the composition includes maleic anhydride or succinic anhydride. The specification states that the anhydride additive improves mixing between the nylon and the rubber compound. Better results are obtained in an example in which maleic anhydride is used, but the properties of the product obtained are not particularly good, and are not adequate for commercial use.
U.S. Pat. No. 4,508,867 relates to vulcanizable rubbery compositions containing a crystalline polyamide, a synthetic rubbery polymer composed of acrylonitrile or methacrylonitrile, an α,β-unsaturated carboxylic acid and butadiene, an additive selected from the halides of lithium, magnesium, calcium and zinc, an additive selected from the oxides and hydroxides of magnesium, calcium, barium and zinc and the peroxides of calcium and zinc and further contains sulfur vulcanization active agents. Nylon 11 is the only polyamide whose use is exemplified.
The descriptive portion of the specification suggests that the mixing of the polyamide and the synthetic rubbery polymer should take place at a temperature in the range of from about 50 to about 125° C. In Examples 1 and 2 mixing of nylon 11 and carboxylated nitrile rubber, and other ingredients, took place at 50° C. In Example 3 mixing took place at 190 to 199° C. and Example 4 does not specify the temperature of mixing. It is believed that the compositions of U.S. Pat. No. 4,508,867 do not display adequate heat resistant properties.
WO 03/020820 A1 describes the preparation of heat and oil resistant polymer blends of polyamides and hydrogenated carboxylated nitrile rubber prepared according to a single step process.
Co-pending US Patent Application filed on Dec. 12, 2004 entitled “Multistage Process for the Manufacture of Peroxide Cured HXNBR-Polyamide Thermoplastic Vulcanizates” discloses TPVs having improved morphology and smaller rubber particle size than those previously discovered.